IaaS : Infrastructure as a Service

"Infrastructure as a service" (IaaS), deliver computer infrastructure – typically a platform virtualisation environment – as a service, along with raw storage and networking. Rather than purchasing servers, software, data-center space or network equipment, clients instead buy those resources as a fully outsourced service. Suppliers typically bill such services on a utility computing basis; the amount of resources consumed and the cost will typically reflect the level of activity
Amazon Web Services is one of such services which provides IaaS.